mm/sparse-vmemmap: provide generic vmemmap_set_pmd() and vmemmap_check_pmd()

mm/sparse-vmemmap: provide generic vmemmap_set_pmd() and vmemmap_check_pmd()

Patch series "mm/sparse-vmemmap: Provide generic vmemmap_set_pmd() and
vmemmap_check_pmd()", v3.

The weak vmemmap_set_pmd() and vmemmap_check_pmd() hooks are currently
no-ops in the generic code, which leaves architectures that need PMD-level
handling to open-code the same logic locally.

This series provides generic implementations for both helpers in
mm/sparse-vmemmap.c.  vmemmap_set_pmd() installs a huge PMD with
PAGE_KERNEL protection, and vmemmap_check_pmd() verifies a present leaf
PMD before reusing the existing vmemmap_verify() helper.

With those generic helpers in place, patches 2-5 remove the now redundant
arch-specific implementations from arm64, riscv, loongarch, and sparc.

This patch (of 5):

The two weak functions are currently no-ops on every architecture, forcing
each platform that needs them to duplicate the same handful of lines.
Provide a generic implementation:

- vmemmap_set_pmd() simply sets a huge PMD with PAGE_KERNEL protection.

- vmemmap_check_pmd() verifies that the PMD is present and leaf,
  then calls the existing vmemmap_verify() helper.

Architectures that need special handling can continue to override the weak
symbols; everyone else gets the standard version for free.

Link: https://lore.kernel.org/20260601084845.3792171-1-songmuchun@bytedance.com
Link: https://lore.kernel.org/20260601084845.3792171-2-songmuchun@bytedance.com
Signed-off-by: Muchun Song <songmuchun@bytedance.com>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Acked-by: Oscar Salvador (SUSE) <osalvador@kernel.org>
Cc: Albert Ou <aou@eecs.berkeley.edu>
Cc: Alexandre Ghiti <alex@ghiti.fr>
Cc: Andreas Larsson <andreas@gaisler.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: David S. Miller <davem@davemloft.net>
Cc: Huacai Chen <chenhuacai@kernel.org>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Palmer Dabbelt <palmer@dabbelt.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: WANG Xuerui <kernel@xen0n.name>
Cc: Will Deacon <will@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>